Manufacture of high purity nitric oxide

ABSTRACT

Process, integrated with ammonia air oxidation, for the manufacture of nitric oxide of at least 99 percent purity comprising the steps of (1) effecting the ammonia oxidation, (2) maintaining maximum NO content in the oxidation effluent, and then contacting the effluent with nitric acid to convert the NO to NO2, (3) cooling the NO2 containing gas product to condense the NO2 therefrom as liquid N2O4, (4) passing the liquid N2O4 into a packed converter tower and therein at least partially vaporizing the liquid; passing resulting vaporization product upwardly through the tower packing and passing water or weak nitric acid into the tower and downwardly through the packing in countercurrent flow reaction contact with the vaporization product, and then into the zone of said vaporization and reaction contact with any unvaporized liquid, and vaporization product to convert substantially all of the initially added liquid dinitrogen tetroxide to nitric oxide gas product containing minor contaminant amounts of NO2, HNO3 and water, (5) water-scrubbing the NO2 and HNO3 contaminants from the NO gas product to form resulting water-wet NO gas product containing, on an anhydrous basis, at least 99 percent NO, and (6) dehydrating the water-wet NO gas product to form said NO of at least 99 weight percent purity. Strong nitric acid is formed in the converter tower as by-product.

United States Patent De Vry 1 May 16, 1972 [54] MANUFACTURE OF HIGHPURITY NITRIC OXIDE Frank E. De Vry, Greenville, Del.

[73] Assignee: Hercules Incorporated, Wilmington, Del.

[22] Filed: Sept. 21, 1970 [21] Appl. No.: 74,076

Related 1.1.5. Application Data [63] Continuation-in-part of Ser. No.731,959, May 24,

1968, abandoned.

[72] Inventor:

2,046,162 6/1936 3,ll0,563 ll/l963 Handforth et al.. ....23/l62 X Krausset a1. ..23/l 62 Primary Examiner-Oscar R. Vertiz Assistant ExaminerG.O. Peters AttorneyS. Grant Stewart [57] ABSTRACT Process, integratedwith ammonia air oxidation, for the manufacture of nitric oxide of atleast 99 percent purity comprising the steps of (l) effecting theammonia oxidation, (2) maintaining maximum N0 content in the oxidationeffluent, and then contacting the effluent with nitric acid to convertthe NO to N0,, (3) cooling the N0 containing gas product to condense theNO, therefrom as liquid N 0 (4) passing the liquid N 0, into a packedconverter tower and therein at least partially vaporizing the liquid;passing resulting vaporization product upwardly through the towerpacking and pasing water or weak nitric acid into the tower anddownwardly through the packing in countercurrent flow reaction contactwith the vaporization product, and then into the zone of saidvaporization and reaction contact with any unvaporized liquid, andvaporization product to convert substantially all of the initially addedliquid dinitrogen tetroxide to nitric oxide gas product containing minorcontaminant amounts of N0,, HNQ', and water, (5) water-scrubbing the NO,and HNO, contaminants from the N0 gas product to form resulting waterwetN0 gas product containing, on an anhydrous basis, at least 99 percentNO, and (6) dehydrating the water-wet N0 gas product to form said N0 ofat least 99 weight percent purity. Strong nitric acid is formed in theconverter tower as byproduct.

9 Claims, 1 Drawing Figure MANUFACTURE OF HIGH PURITY NITRIC OXIDE Thisapplication is a continuation-in-part of my copending application Ser.No. 731,959, filed May 24, l968 now abandoned.

This invention relates to the production of nitric oxide and moreparticularly to an integrated process for the production of anhydrousnitric oxide of high purity.

The potential of nitric oxide as a widely used chemical product has longbeen realized. However, the economic burdens of conventional preparationof the product have, to a considerable extent, retarded more generalusage.

Now, in accordance with the present invention, it has been found thathigh yields of nitric oxide having a purity of at least 99 percent canbe obtained based upon a unique combination of processing steps whichgives a high yield of valuable byproduct when desired, whereby theoverall system attains a new level of economic attractiveness.

Accordingly, a primary object of the present invention contemplates asingle, overall process in which a basic ammonia oxidation process forthe production of nitric acid is integrated with the successivesynthesis of nitrogen tetroxide and its conversion to nitric oxide toyield a nitric oxide end product of at least 99 percent purity and onlya single byproduct; that is, strong nitric acid having a concentrationof more than about 76 percent. Such high purity nitric oxide is becomingincreasingly important in its own right and as a raw material for themanufacture of N 0,, and NOCl. Thus, the attainment of the presentobject serves to considerably accelerate the importance of this product.

Other objects of the invention will appear hereinafter the novelfeatures and combinations being set forth in the appended claims.

Generally stated, this invention is an overall process for themanufacture of nitric oxide product containing at least 99 weightpercent NO, beginning with the production of nitric oxide as part of amixture of reaction products resulting from the air oxidation ofammonia. The nitric oxide thus formed is first reacted, as a componentof the total ammonia oxidation efiluent, with nitric acid, to convert itto nitrogen dioxide, with subsequent separation of the nitrogen dioxideas dinitrogen tetroxide condensate. The liquid dinitrogen tetroxide isthen recovered and converted back to nitric oxide by reaction, in atleast partially evaporated form, with water (as such or in the form ofweak nitric acid). Small contaminant amounts of nitrogen dioxide andnitric acid are water-scrubbed from the nitric oxide product, and theresulting wet nitric oxide is dehydrated to yield the nitric oxide endproduct. Nitric acid is formed during the nitric oxide forming step andis removed from the process as strong acid by-product.

The tail gases from the dinitrogen tetroxide condensation, which includeminor amounts of entrained nitric oxide and nitrogen dioxide, areconducted to a conventional oxidation and absorption tower where theremaining oxides of nitrogen are converted to nitric acid; and the waterformed during the nitric oxide conversion to nitrogen dioxide is removedfrom the system in form of diluted nitric acid which may be used as suchor concentrated in a vacuum still. A method for preparing liquiddinitrogen tetroxide, starting with air oxidation of ammonia, isdisclosed in US. Pat. No. 3,063,804 to D. G. Morrow. The term "nitrogendioxide as used herein connotes NO,, N 0, or such equilibrium mixture ofNO, and N 0, as may exist under the conditions prevailing in each stepof the process.

More specifically, the invention provides an overall process integratedwith conventional ammonia air oxidation for the manufacture of at leastabout 99 percent purity nitric oxide, which comprises air-oxidizingammonia in the presence of a stoichiometric excess of air to form, astotal effluent, a gas mixture containing nitric oxide as substantiallythe only nitrogen oxide product; maintaining said total effluent at atemperature sufficiently high to retain substantially all of said nitricoxide product therein, and countercurrently contacting said efi'luent,initially at said temperature, with downwardly flowing nitric acid toreact said nitric oxide with said nitric acid to convert said nitricoxide to nitrogen dioxide, and thereby form resulting nitrogen dioxidegas product containing nitrogen and oxygen components of said excessair; cooling said nitrogen dioxide gas product sufficiently to condensesubstantially all of said nitrogen dioxide therefrom as liquiddinitrogen tetroxide without condensation of said nitrogen and oxygencomponents; introducing said liquid dinitrogen tetroxide into anupwardly extending tower containing a packed section along its length,and then vaporizing at least a portion of said liquid; passing resultingvaporization product upwardly through said packed section, and passingwater or weak nitric acid into said tower and then downwardly throughsaid packed section in countercurrent flow reaction contact with saidupwardly flowing vaporization product and then into the zone of saidvaporization in reaction contact therein with any unvaporized portion ofsaid liquid nitrogen tetroxide and vaporization product to convertsubstantially all of said liquid dinitrogen tetroxide, initiallyintroduced into said tower, to nitric oxide and thereby form nitricoxide gas product containing small contaminant amounts of water,nitrogen dioxide and nitric acid; passing said nitric oxide gas productfrom said tower, and water-scrubbing substantially all of said nitrogendioxide and nitric acid contaminants therefrom to form resultingwater-wet nitric oxide containing, on a substantially anhydrous basis,at least 99 weight percent NO; and dehydrating said wet nitric oxide toform nitric oxide product containing at least 99 weight percent NO.Strong nitric acid is withdrawn from the tower as by-product of thenitric oxide forming reaction.

The ammonia oxidation step is conducted under well known hightemperature catalytic conditions such as, for example, utilizing anammonia to air gas volume ratio of from about 9:1 to l l:l, a suitableammonia oxidation catalyst such as percent platinum-IO percent rhodiumat a catalyst temperature of about 900 to l,200 C., and a pressure ofabout 0 to 200 p.s.i.g., to produce nitric oxide in accordance with theequatlon 4Nl-l, 50 4N0 6H,0 (I) It is an important requirement thatsubstantially all nitric oxide fonned during the oxidation be availablefor the subsequent reaction with nitric acid to form nitrogen dioxide,in order to accomplish formation of nitric oxide end product in maximumyield. lt is therefore necessary that until the total gas effluent fromthe oxidation is introduced into the nitric acid reaction to formnitrogen dioxide, it be maintained at a sufiiciently high temperature,generally at least 600 F., to substantially preclude loss of nitricoxide product to reaction with the excess oxygen present, which wouldotherwise occur in accordance with the equation The nitric acid, nitricoxide reaction to convert the nitric oxide to nitrogen dioxide proceedsin accordance with the well known equilibrium,

NO ZHNO, 3N0, H 0 (3) generally utilizing nitric acid containing atleast 50 weight percent l-lNO,, at a temperature within the range offrom l50 to 225 F. and a pressure within the range of from atmosphericto I50 p.s.i.g.

The high yield of nitric oxide end product is accomplished in practiceof the invention by first reacting the nitric oxide product of ammoniaoxidation with nitric acid to obtain 3 moles nitrogen dioxide per moleof nitric oxide initially formed during the ammonia oxidation, therebyproviding three times the amount of nitrogen dioxide available forcondensation that would be obtained by conventional cooling of theammonia oxidation effluent and direct recovery of the nitrogen dioxideformed. The combination of steps involving maintaining minimumtemperature of the ammonia oxidation effluent and reaction of nitricacid with all nitric oxide retained therein, is therefore critical tothe accomplishment of high yield of nitric oxide end product.

In order that the end nitric oxide product, which contains at least 99percent N0, be accomplished, the nitrogen dioxide formed during thenitric oxide, nitric acid reaction must be substantially free fromdissolved or entrained nitrogen and oxygen from the ammonia oxidation.Conventional fractionation of the gas efiluent from the nitric acid,nitric oxide reaction to produce nitrogen dioxide of the requisitepurity for sub sequent conversion to nitric oxide is undesirable fromthe standpoint of time, power, equipment and manpower requirements. Suchis made unnecessary in accordance with the invention and is accomplishedby cooling the total gas effluent from the nitric acid, nitric oxidereaction to condense the nitrogen dioxide therefrom as liquid nitrogentetroxide, under which conditions none of the other gas effluentcomponents, except water, undergo condensation, and are thereforereadily separated from the condensate as tail gases for furtherprocessing. Generally, cooling of the total effluent gas from the nitricoxide, nitric acid reaction is accomplished in a plurality of stages atprogressively lower temperature levels.

The total gas effluent emerging from the nitric oxide forming stepcontains nitric oxide end product with minor contaminant amounts ofunreacted nitrogen dioxide, weak nitric acid and water vapor. Thenitrogen dioxide and nitric acid contaminants are water-scrubbedsubstantially free of nitrogen dioxide and nitric acid contaminantsunder ambient temperature conditions, and residual "wet" nitric oxidestream is subsequently dehydrated generally by contact with soliddesiccant to provide the nitric oxide end product of the invention.

The liquid dinitrogen tetroxide introduced into the packed tower isreactable with the water or weak nitric acid as nitrogen tetroxide, ornitrogen dioxide, or both, depending on local temperature and pressureconditions. However, to the extent that the liquid nitrogen tetroxide isreacted, as such, substantially all reaction with the water or weaknitric acid takes place in the tower section adjacent the point ofingress of the liquid reactant.

The invention will be illustrated in greater detail with reference tothe accompanying diagrammatic drawing in which the description is anexample of plant operation of the invention and the percentages givenare by weight unless otherwise designated. The rate amounts givencorrespond to the production of tons of nitric oxide per 24-hour day.Although the following description has been divided into several stepsfor the purpose of clarity, it will be understood that the overallprocess is integrated and may be operated as a continuous process.

STEP 1 PRODUCTION OF CRUDE DINITROGEN TETROXIDE Anhydrous ammonia fromconduit 2 at the rate of 10.2 lbs/min. is mixed with preheated air fromconduit 4 to make a mixture containing 10.3 percent NH, by volume whichis then passed over a platinum-rhodium catalyst in a conventionalammonia oxidation converter 6 at approximately 1 l0 p.s.i.g. to form anoxidation product mixture having the following composition by volume:

N, 12.1% N0 10.7 11,0 10.1 o, 7.1

The temperature of this gas mixture is maintained between 660 F. and 700F. by means of a cooling jacket on conduit 8 (not shown) and the streamis conducted under those temperature conditions as quickly as possibleto a nitrogen tetroxide reactor 10 to thereby retain thereinsubstantially all nitric oxide formed during the oxidation. Here the hotgas stream in reactor 10 is countercurrently contacted by 170 lbs./min.of 63,5 percent nitric acid, as fed through conduit 12 under whichconditions the nitric oxide component of the gas stream is converted tonitrogen dioxide by reaction with the nitric acid in accordance with thewell known equilibrium as shown in equation 3 above. The nitric acid isused in approximately 300 percent excess over that stoichiometricallyrequired for the reaction, the surplus serving as a heat sink and alsoto allow even wetting of the packing which occupies the reaction zone ofthe reactor 10. The operating pressure in the reactor 10 is maintainedat about p.s.i.g. by regulation of the flow of total spent gas effluent.This spent gas flow rate is 128 lbs/min. and the spent gas is of thefollowing composition:

The water formed in the reaction acts to dilute the excess nitric acidto weak acid, which is allowed to exit from the bottom of nitrogentetroxide reactor 10 through conduit 14 to a level regulator and acidcooler 16. The weak acid rate is 164 lbs/min. and the strength of theweak acid is 50 percent. It is fed from acid cooler 16 through conduit18 to an oxidation tower 20 for concentration to 61 percent strength.

A crude nitrogen tetroxide product containing approximately 1.5 percentwater is continuously condensed from the overhead gas stream leaving thenitrogen tetroxide reactor 10. This is accomplished by means of apre-coo1er 22 followed by a brine-cooled main and vent condensers 24 and26, connected by conduits 28, 30 and 32 respectively, operating at 22 F.and 15 F. respectively. The rate of condensation is 43.8 lbs/min. and ispassed from condensers 24 and 26 through conduits 34 and 36 into conduit38. Of this amount, 12.3 lbs/min. is returned to the reactor 10 throughconduit 40 as reflux and 31.5 lbs/min. removed as crude liquiddinitrogen tetroxide through the conduit 38 and conducted to a storagetank 42.

STEP ll PRODUCTION AND DEHYDRATION OF NITRIC OXIDE Nitric oxide isproduced by reversing the reaction by which nitrogen dioxide is formedin reactor 10, as shown by the equation 3N0 H,O N0 2HN0 (4) This isaccomplished in a packed reaction tower operating at from atmospheric toabout p.s.i.g. and at a temperature range offrom about 150 to about 225F.

The nitrogen tetroxide produced in the preceding step (3 l .5 lbs/min.)is fed from the storage tank 42 through conduit 44 into the nitric oxidereaction tower 46 as a liquid and at a point just below the packingsupport, wherein it is at least partially vaporized to form nitrogendioxide including a minor proportion of nitrogen tetroxide. The waterfor the reaction is supplied partly from the water in 18.1 lbs/min. of58 percent nitric acid which is pumped through conduit 48 into thereaction tower 46 and over the reactor packing countercurrent to theflow of nitrogen dioxide and partly from 1.9 gals/min. of water fedthrough conduit 50 into scrubber 52 at the top of tower 46 used to scrubthe nitric oxide product free from entrained nitrogen dioxide and nitricacid. Thus, the nitric acid undergoes a concentration as well as anincrease in total quantity as it passes down the tower 46 and passesfrom the bottom thereof as 45.8 lbs/min. having a strength approaching88 percent. This acid is then passed through conduit 54 into a steamheated bleacher 56 and heated to 215 F. to remove dissolved nitrogendioxide with the overhead nitrogen dioxide passed through conduit 58into the bottom of tower 46. The acid from the bottom of bleacher 56 ispassed through conduit 60 into a cooler 62 and is passed therefromthrough conduit 64 and is then divided into two streams. The firststream of 22.7 lbs/min. is returned through conduit 66 to the weaknitric (58 percent) storage tank 68 while the second stream of 22.0lbs/min. is withdrawn from the system through conduit 70 as an exportstream of by-product 88 percent nitric acid.

The effluent gaseous product nitric oxide issues from the top of thetower 46 at the rate of 7.0 lbs/min. and has the following analysis byvolume:

The water is removed by passing this wet gas through conduit 72 intodesiccators 74 and 76 containing activated alumina. The final anhydrousnitric oxide product contains about 99.0 percent NO and about 1.0percent NO, and is withdrawn from the system through conduit 78.

STEP Ill REMOVAL OF WATER FROM THE PROCESS An inspection of the overallwater valance shows that l6.6 lbs/min. of water must be removed from theprocess as such. This is accomplished in a vacuum still 80 which is fedthrough conduit 82 at the rate of 186.0 lbsJmin. of 58 percent nitricacid from storage tank 68 and concentrating it to 169.4 lbs/min. of 63.5percent nitric acid with removal of 16.6 lbs/min. of water ascondensate. The still 80 operates at an average pressure of 135 mm. Hgabsolute and the bottom product therefrom is passed through conduit 84to a cooler 86 and therefrom through conduit 88 to the strong nitric(63.5 percent) storage tank 90. The overhead product from the still 80is passed through conduit 92 to a cooler 94 and therefrom throughconduit 96 to a water jet 98 and then to sump conduit [00. Moreover,overhead product from the vent condenser 26 is passed through conduit102 with injection of air through line 104 into the bottom of oxidationtower 20. The weak acid from line 18 also passes into the bottom oftower 20. Water through conduit 106 is fed to the top of the tower 20incounterflow to the gaseous products. The nitric acid of 61 percentstrength heretofore referred to is withdrawn from the bottom of thetower 20 and passed through conduit )8 to the weak nitric storage tank68. Since weak nitric acid of 50 percent strength is passed from conduit18 through conduit 110 into storage tank 68 from about l to aboutpercent of the strong acid from line 66 is added thereto and theresulting fortified acid is of 58 percent strength for pumping to thetower 46 and to the vacuum still 80 through conduits 48 and 82,respectively.

From the above example, it will be seen how the several steps arecombined into a single, unified and continuous process for theproduction of anhydrous nitric oxide. The process lends itself well tointegration with an existing high pressure nitric acid plant. In thiscase it has the advantage of allowing the concentration of nitric acidabove the usual normal limit of 63 percent obtained from the typicalammonia oxidation nitric acid process. The 88 percent nitric acidobtained according to the above example is sufficiently strong for manyindustrial uses. It can, however, be further concentrated in a separatevacuum still to 99 percent, since the starting acid strength is abovethe 65 percent azeotrope limit.

in instances where there is no economic advantage in having ahigh-strength stream of by-product export nitric acid, the high strengthacid may be used as part of the acid feed to the nitric acid-nitricoxide reactor. This has two advantages: l it will increase theefficiency of the nitric oxide conversion by increasing the strength ofthe feed nitric acid and (2) it will relieve some of the water removalload from the nitric acid vacuum still.

Thus, it is evident that there are several factors which will influenceconditions for the most satisfactory operation of the invention, theactual requirements of which are determined by the end product andby-product involved. Although the embodiment of the invention describedgave a by-product of high strength 88 percent nitric acid, it will beappreciated that the process may be attractively practiced to obtainanhydrous nitric oxide of high purity as the end product with highstrength niric acid as by-product having a concentration of from about76 to about 88 percent. Moreover, it will be appreciated that thisstrong by-product acid may be obtained when the process is operated inconjunction with weak nitric acid having a concentration of from about40 to about 62 percent and even when the acid concentration is as low as5 percent.

lt will be seen, therefore, that this invention may be carried out bythe use of various modifications and changes without departing from itsspirit and scope, with only such limitations placed thereon as areimposed by the appended claims.

What I claim and desire to protect by Letters Patent is:

l. A process integrated with conventional ammonia air-oxidation for themanufacture of at least about 99 percent purity nitric oxide, whichcomprises air-oxidizing ammonia in the presence of a stoichiometricexcess of air to form, as total effluent, a gas mixture containingnitric oxide as substantially the only nitrogen oxide product;maintaining said total effluent at a temperature sufficiently high toretain substantially all of said nitric oxide product therein, andcountercurrently contacting said effluent, initially at saidtemperature, with downwardly flowing nitric acid to react said nitricoxide with said nitric acid to convert said nitric oxide to nitrogendioxide, and thereby form resulting nitrogen dioxide gas productcontaining nitrogen and oxygen components of said excess air; coolingsaid nitrogen dioxide gas product sufi'iciently to condensesubstantially all of said nitrogen dioxide therefrom as liquiddinitrogen tetroxide without condensation of said nitrogen and oxygencomponents; introducing said liquid dinitrogen tetroxide into anupwardly extending tower containing a packed section along its length,and then vaporizing at least a portion of said liquid; passing resultingvaporization product upwardly through said packed section, and passingwater or weak nitric acid into said tower and then downwardly throughsaid packed section in countercurrent flow reaction contact with saidupwardly flowing vaporization product and then into the zone of saidvaporization in reaction contact therein with any unvaporized portion ofsaid liquid nitrogen tetroxide and vaporization product to convertsubstantially all of said liquid dinitrogen tetroxide, initiallyintroduced into said tower, to nitric oxide and thereby form nitricoxide gas product containing small contaminant amounts of water,nitrogen dioxide and nitric acid; passing said nitric oxide gas productfrom said tower, and water-scrubbing substantially all of said nitrogendioxide and nitric acid contaminants therefrom to form resultingwater-wet nitric oxide containing, on a substantially anhydrous basis,at least 99 weight percent N0; and dehydrating said wet nitric oxide toform nitric oxide product containing at least 99 weight percent N0.

2. In a process of claim 1, introducing said liquid dinitrogen tetroxideinto an open lower section of said tower subjacent said packed section,whereby said zone of vaporization is substantially in said lower towersection; introducing said water or weak nitric acid into an upper opensection of said tower adjacent and above said packed section; andpassing said nitric oxide gas product from said tower at a point abovethe ingress of said water or weak nitric acid into said upper towersection.

3. in a process of claim 2, water-scrubbing said nitric oxide product bypassing same from said upper tower section into countercurrent flowcontact with water, to thereby water-absorb substantially all of saidnitrogen dioxide and nitric acid contaminants from said nitric oxide gasproduct; and passing residual water from said scrubbing contact intosaid upper tower section for flow through said packed section togetherwith said water or weak nitric acid separately passed into said tower.

4. In a process of claim 3, passing nitric acid containing from about 40to about 62 weight percent HNO, into said upper tower section for saidcountercurrent flow reaction contact to form said nitric oxide, andrecovering strong nitric acid by-product containing from about 76 to 88weight percent HNO 5. in a process of claim 4, withdrawing total acidfrom said tower at a point below the tower ingress of said liquid formsaid nitric oxide having a concentration of about 58 weight percent N03.

8. A process in accordance with claim 1 in which said tower is operatedat from atmospheric to about p.s.i.g. and at a temperature of from about150 to 225 Fr 9 In a process of claim 4, said nitric oxide productcontaining about 99 percent nitric oxide and about I percent nitrogendioxide.

I I i i

2. In a process of claim 1, introducing said liquid dinitrogen tetroxideinto an open lower section of said tower subjacent said packed section,whereby said zone of vaporization is substantially in said lower towersection; introducing said water or weak nitric acid into an upper opensection of said tower adjacent and above said packed section; andpassing said nitric oxide gas product from said tower at a point abovethe ingress of said water or weak nitric acid into said upper towersection.
 3. In a process of claim 2, water-scrubbing said nitric oxideproduct by passing same from said upper tower section intocountercurrent flow contact with water, to thereby water-absorbsubstantially all of said nitrogen dioxide and nitric acid contaminantsfrom said nitric oxide gas product; and passing residual water from saidscrubbing contact into said upper tower section for flow through saidpacked section together with said water or weak nitric acid separatelypassed into said tower.
 4. In a process of claim 3, passing nitric acidcontaining from about 40 to about 62 weight percent HNO3 into said uppertower section for said countercurrent flow reaction contact to form saidnitric oxide, and recovering strong nitric acid by-product containingfrom about 76 to 88 weight percent HNO3.
 5. In a process of claim 4,withdrawing total acid from said tower at a point below the toweringress of said liquid dinitrogen tetroxide, and then heating thewithdrawn acid to remove any dissolved nitrogen dioxide therefrom;returning nitrogen dioxide thus removed from said acid to saidcountercurrent contact with said weak nitric acid; and recoveringresulting residual acid as said strong nitric acid by-product.
 6. In aprocess of claim 4, contacting said ammonia oxidation effluent withnitric acid containing at least 50 weight percent HNO3 to form saidnitrogen dioxide gas product.
 7. In a process of claim 4, said nitricacid for reaction to form said nitric oxide having a concentration ofabout 58 weight percent HNO3.
 8. A process in accordance wiTh claim 1 inwhich said tower is operated at from atmospheric to about 150 p.s.i.g.and at a temperature of from about 150* to 225* F.
 9. In a process ofclaim 4, said nitric oxide product containing about 99 percent nitricoxide and about 1 percent nitrogen dioxide.